1. Field of the Invention
The present invention relates to a resonator, which filters an oscillation or wave of a predetermined frequency using a resonance phenomenon, an apparatus having the same and a fabrication method of the resonator.
2. Description of the Related Art
With the recent rapid development of mobile communication apparatuses, chemical and bio apparatuses, etc., a demand for a small, lightweight filter, an oscillator, a resonant element, an acoustic resonant mass sensor, a duplexer, etc., which are used in such apparatuses, is increasing.
As means for realizing the small, lightweight filter, the oscillator, the resonant element, the acoustic resonant mass sensor, the duplexer, etc., a film bulk acoustic resonator (FBAR) is known.
In general, the FBAR is configured in a structure including a resonating unit in which a lower electrode, a piezoelectric film, and an upper electrode are layered in turn on a substrate.
The FBAR constructed as above forms a part for electronic circuit, such as a filter, an oscillator, a resonant element, an acoustic resonant mass sensor, a duplexer, etc., out of a single FBAR or a plurality of FBARs combined with each other.
For example, if a plurality of FBARs are properly combined in series and in shunt, a band-pass filter with a predetermined central frequency and a predetermined frequency band width can be embodied. Also, if the band-pass filter is combined with a phase shifter made up of an inductor and a capacitor, a duplexer can be embodied.
In such a duplexer, a filtering performance is influenced by a roll-off characteristic of the band-pass filter.
For instance, in case of a US personal communication system (US-PCS) duplexer, as illustrated in a dotted line of FIG. 1, a roll-off of a transmitter filter Tx and a receiver filter Rx is located in a band of 1915˜1925 MHz (10 MHz) that a band of ±3 MHz (6 MHz) under the influence caused by a temperature coefficient TC effect and a process margin band of ±2 MHz (4 MHz) taking account of thickness uniformity are subtracted from a transmitter filter-receiver filter guardband of 1910˜1930 MHz (20 MHz). Such a roll-off is located in a very narrow band, as illustrated in a waveform diagram of FIG. 2, which explains an effective frequency of the transmitter filter Tx under the influence caused by the temperature coefficient TC effect.
Accordingly, in an operation of a FBAR forming a filter, to prevent a resonance frequency of the FBAR from interfering with the effective frequency of the filter, it is important that the FBAR should be fabricated so as to get a value of an effective piezoelectric coupling coefficient Kt2_eff thereof to satisfy a roll-off of the filter. Here, the effective piezoelectric coupling coefficient Kt2_eff of the FBAR, which shows a ratio of electric energy to acoustic energy in the operation of the FBAR, can be represented as a width between a peak point of a resonance frequency and a peak point of an antiresonance frequency, as illustrated in FIG. 3.
Thus, there is needed a new FBAR capable of freely adjusting the effective piezoelectric coupling coefficient Kt2_eff of the FBAR so as to satisfy the roll-off of the filter.